To supply electric power from an AC power source to a load efficiently, a resonance type non-contact power supply system must transmit the electric power from the AC power source efficiently to a resonance system. If the resonance type non-contact power supply system has a constant distance between the power supply side (the primary side) and the power reception side (the secondary side) and the load connected to the power reception side is invariable, a test is conducted initially to determine the output frequency of the AC power source, which is the resonance frequency of the resonance system. The AC power source outputs AC voltage to a primary coil at the obtained output frequency.
However, if the distance between the resonance coils varies or impedance changes in the load, the input impedance of the resonance system at the resonance frequency of the resonance system also changes. This hampers matching between the AC power source and the input impedance of the resonance system, thus increasing the reflected electric power to the AC power source. In other words, the load cannot receive the electric power efficiently.
When the resonance type non-contact power supply system is used to charge a battery, the load varies in correspondence with the charging state of the battery. This also changes the input impedance of the resonance system, resulting in increase in the reflected electric power to the AC power source.
One such conventionally proposed power transmission system improves efficiency for electric power transmission in a wide range of communicability distances for a non-contact IC card system performing electric power transmission and signal communication by electromagnetic induction (see Patent Document 1). The power transmission system of Patent Document 1 includes a set of an electromagnetic field generating device and an electromagnetic field receiving device, which perform electric power transmission and signal communication by electromagnetic induction. At least one of the electromagnetic field generating means of the electromagnetic field generating device and the electromagnetic field receiving means of the electromagnetic field receiving device includes variable impedance matching means for variably controlling input-output impedance and control means for controlling the variable impedance matching means. The control means controls the input-output impedance of the variable impedance matching means to match impedance between the electromagnetic field generating device and the electromagnetic field reception device. This increases the electric power received by the electromagnetic field receiving device, according to Patent Document 1.